The Physics
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Diameter of a Raindrop

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Bibliographic Entry Result
(w/surrounding text)
"Precipitation." Earth Science. Illinois: Heath, 1999. "A raindrop may have a maximum diameter of 0.25 centimeter." 2.5 mm
"Rain." Encyclopedia Encarta.1st ed. CD-ROM. New York: Microsoft, 2000. "Raindrops generally have a diameter greater than 0.5 mm (0.02 in.). They range in size up to about 3 mm (about 0.13 in.) in diameter." 0.5–3 mm
Davis, Neil T. "Raindrop Size Article #236." Alaska Science Forum. 28 June 1978. "The 4 mm maximum diameter of raindrops probably results because raindrops larger than this size tend to break up when colliding with other large raindrops." < 4 mm
"Characteristics of Particles and Particle Dispersoids." Handbook of Chemistry and Physics. 62nd Edition. New York: CRC, 1981. [chart] 0.5–10 mm
Formation of Raindrops. "Raindrops vary in size from about 0.02 in. (0.5 mm) to as much as 0.33 in. (8 mm) in thunderstorms." 0.5–8 mm

Raindrops are common occurrences that everyone has experienced. However, not many people actually know the real size of a raindrop and the scientific principles behind this simple tiny glob of matter. A raindrop occurs when the water vapor from a cloud wraps itself around tiny particles during condensation (the process by which a gas turns into a liquid). Contrary to popular opinion, raindrops are not shaped like teardrops. In fact, they are actually oblate spheroids, or spheres with the nose smashed in.

Different sources approximate different ranges for the measure of a diameter of a raindrop. However, on the average, a raindrop is between 0.1 to 5 millimeters. There are some exceptions; rarely, raindrops of 8 millimeters were known to occur. Sizes larger than that do not normally occur because the raindrop particles simply break up or collide with other neighboring particles. The appearance of large raindrops always signals strong updrafts and turbulence.

Presently, precipitation is believed to be triggered by a course of action called the collision-coalescence process. Most cloud droplets are extremely small that the motion of the air keeps them suspended. Because large cloud droplets fall much faster than smaller droplets, they are able to sweep up the smaller ones in their path and thus grow and expand in size and volume.

Several important factors affect the diameter or size of a raindrop. First, the fall velocity of a raindrop particle is directly proportional to its diameter. The larger the particle, the faster it falls. The same follows for the maximum fall distance before evaporation, or the process in which a liquid turns into a gas. The larger the diameter, the greater the distance it will fall due to gravity, the force that pulls a water droplet toward the earth's surface.

As a droplet falls, it also encounters air resistance or frictional force. The magnitude of this force depends on the size of the drop's "bottom", or the surface area resisting the fall. Frictional drag increases as the particle accelerates, or speeds up. The frictional and gravitational forces finally balance and the droplet falls at a constant speed, called the terminal velocity. This again depends on size; smaller droplets have a lower terminal velocity than larger droplets.

Igor Volynets -- 2001

Bibliographic Entry Result
(w/surrounding text)
Heidorn, Keith C. Philipp Lenard: Brushing the Teardrops from Rain. The Weather Doctor. 1 July 2000. "Since Lenard found no drops with diameters less than 0.5 mm (0.02 inch), he did conclude that the updrafts in the clouds must be of sufficient strength to prevent such small drops from falling out. We also know that he only recorded one drop in the 4.75 to 5.25 mm diameter range." 0.5 mm
"By suspending drops of known size, he determined that small drops up to about 2 mm (0.08 inches) in diameter "fell" as spheres. Larger drops, however, deformed while falling acquiring a shape with a flat bottom and rounded top similar to that of a hamburger bun. Thus, Lenard was the first to report that raindrops were not the stereotypical teardrop shape but were spherical when small and shaped much like a hamburger bun when larger." < 2 mm
> 2 mm
"Drops, however, became unstable at diameters greater than 5.5 mm (0.21 inches), Lenard found. They lasted less than a few seconds before breaking apart in the airflow, torn asunder by the aerodynamic forces acting on the drop. This observation combined with the lack of drops larger than this size in his rainfall measurements led Lenard to conclude that the maximum drop size possible in nature was just larger than 5 mm." > 5.5 mm
~ 5 mm
Heidorn, Keith C. Wilson A. Bentley: The Raindrop Man, Too. The Weather Doctor. 1 July 2000.
Pellet Size Category Diameter Range Percent of Total
(867) Drops
Very Small < 0.033 in.
< 0.85 mm
Small 0.033–0.055 in.
0.85–1.4 mm
Medium 0.067–0.125 in.
1.7–3.2 mm
Large 0.143–0.200 in.
3.6–5.1 mm
Very Large > 0.200 in.
> 5.1 mm
0.85 ~ 5.1 mm

Editor's Supplement -- 2001